The present invention relates to cables and in particular to halogen free cables with improved flame/fire resistance and oil/abrasion resistance.
Conventional flame retardant cables are usually made with halogen containing materials like polyvinylchloride (PVD), polychloroprene (PCP), chlorosulphonated polyethylene (CSP), etc. In a fire these materials give off thick, dense and corrosive smoke. In some cable installations this is not considered a drawback, but in sites like oil production platforms, hospitals, telephone exchanges and the like, such reactions are not acceptable. Cables containing halogen free materials have, however, also been known for some time.
A cable having an insulation which, when it is subjected to very high temperatures or even flames, is transformed into an electrically insulating ash, may maintain its working capacity during and after a fire. An assumption is that the insulating ash is kept safe in position within the cable, and also that the cable is fastened securely to constructions which are stable during the fire to avoid bending and mechanical stress in the cable.
Even when all such assumptions are fulfilled, known cables have evidenced a high percentage of failures during flame tests.
One of the main problems encountered in connection with installation of cables in potential fire areas is that the cable insulation may collapse during a fire so that the conductor(s) will shortcircuit to the metal screen. This problem and several solutions have been described in U.S. Pat. Nos. 3,180,925 and 4,280,016.
The cables concerned are usually built up from one or more individually insulated conductors and/or optical fibers, which are embedded in a filler sheath of halogen-free material. The cables also include a braided metal screen as well as outer protective covers.
One theory of how the failures occur in such conventional cables when subjected to high temperatures is as follows: As the insulation and the inner sheaths are burned to ashes, the cable diameter tends to increase slightly. The braid which is applied around the inner sheath, is then radially expanded, and this again results in a longitudinal contraction of the braid. As this longitudinal contraction occurs simultaneously with the radial expansion, the frictional forces between the burned insulation and the multiwire conductor elements become very large. Therefore, the longitudinal contraction of the braid will be frictionally transferred to the helical, outer elements of the multiwire conductor and further to the center wire. Because of their helical form, the outer elements of the multiwire conductor are free to contract in the longitudinal direction. The center wire, on the other hand, in a conventional multiwire conductor, is a straight element, and a contraction in longitudinal direction beyond the elasticity limit, is impossible. Instead, there will be built up a longitudinal compression force until the center wire kinks in an uncontrolled manner. The other elements of the multiwire conductor will then also be forced out in a kink and may, if a large kink occurs, make contact with the metal braid, which finally results in a short circuit of the cable. Uncontrolled kinks as described occur mostly in cables and conductors with cross-sectional conductor areas in the order of 0.5 to 2.5 mm.sup.2 and when the number of conductors within the cable is low (10).